The 21-Day Plan

Sure, we're suckers for a workout that promises flat abs and a tight butt as much as the next girl. But there's a more important reason to exercise: It makes you feel great. "Good mental health is just as important as good physical health, and exercise is crucial to getting both," says trainer Jeanette Jenkins, author of The Hollywood Trainer Weight-Loss Plan: 21 Days to Make Healthy Living a Lifetime Habit. Her plan celebrates all of the fab stuff that comes with exercise: more energy, better health, and the motivation to keep coming back for more. So while this routine will definitely help you see an improvement in the right-around-the-corner skimpy-outfits season, it'll also help you love the shape you're in, inside and out.

How the Plan Works

This 21-day plan is designed for all fitness levels, so follow the exercise and cardio variations that are right for you. Lacing up after a long layoff? Use the three weeks to make exercise a lifelong habit. If you're already in the swing of things, there are plenty of challenges here to boost your results.

The program consists of four primary workouts: three strength circuits and a cardio routine. Each circuit focuses on a different set of muscle groups; the cardio plan is designed to blast calories and improve your aerobic fitness. For best results, do the workouts in the order given below.

Your 21-Day Makeover Plan

Circuit A

Chest, Triceps, and Rear
Do this circuit 4 times through (advanced, do it 6 times)

1. Push-Up/Plank Combo

Targets shoulders, triceps, chest, abs

Begin in a modified push-up position, hands on ground slightly wider than shoulders and knees down. Keep a straight line from the top of your head through your legs, abs tight. Bend elbows 90 degrees, then push back up to start.

Do 20 to 25 reps. (Not shown.)

After your last rep, lift your knees off the ground so you're in a full push-up position with legs extended. Keeping abs tight and back straight, hold for 30 seconds.

Lean back slightly, balancing on your tailbone, and open arms wide out to sides. Hold for 8 breaths, return to start, and repeat. Do 3 reps.

Challenge yourself: Straighten legs about 45 degrees to ground.

Calorie-Blasting Cardio Workout

These treadmill workouts are designed to blast 300 to 500+ calories in about 40 to 50 minutes. Choose from one of three levels (beginner, intermediate, advanced) and pick either a high-impact run or a low-impact speed walk on an incline. Don't have access to a treadmill? Do the same program outside on flat or hilly terrain. Follow your cardio workout with 5 to 10 minutes of stretching for your shoulders, chest, hips, glutes and legs.